Why are there two vernier scales on a prism spectrometer? Why are there two vernier scales on a prism spectrometer and why are they 180 degrees apart?
Example image (source):

I have some idea that it reduces the error in measurements but I don't exactly know how it does that.
this article reasons as follows:
"Record both VERNIER readings (in minutes). Average the two vernier readings (to eliminate any systematic error from misalignment of the circle scale with respect to bearing axis), and add the result to one of the angle scale readings."
can someone elaborate this reasoning for me ?
 A: As pointed out in the comments, the short answer to your question is: To minimize or counter the errors produced in the case when the axis of rotation of telescope and prism table/vernier table do not coincide.
Long Explanation: Assume that the point $O$ in the given figure represents the center of the prism/vernier table of the spectrometer, through which the axis of rotation passes. And assume that the point $O'$ represents the point of intersection of the axis of rotation of the telescope and the plane of the table. Ideally, $O$ and $O'$ should coincide but say, due to some fault in the bearings they displace a little.

In the ideal case, when $O$ and $O'$ do coincide, angle $\theta$ is subtended by $O$ or $O'$ on either of vernier scales ($V_1$ or $V_2$) is equal and thus only one vernier scale will be enough to the job.
In case the two points of rotation displace apart i.e. do not coincide with each other, $O$ (point of rotation for vernier table) will still subtend equal angle $\theta$ on both vernier scales but $O'$ (point of rotation for the telescope) will not because $\alpha$ and $\beta$ will differ and this introduces an error in the readings.
To counter the error introduced by displacement of points of rotation, we take the mean of both the angles $\alpha$ and $\beta$ and thus we use two verniers instead of just one.
EDIT:
In the case when $O$ and $O'$ coincide the angles differ by 180° but when they do not coincide, $\alpha$ and $\beta$ do not differ by 180°.
A: Construction
Construction may vary, this version goes with image along the question.
The vernier scale and the prism table move together on an axis different from that of the main scale, which moves with the telescope.
We are assuming that both the scales move independent of each other about their axes.

The figure presented here is a simplified bird's eye view of the two scales. (circular purple inner scale and main scale )
NOTE : The offset is exaggerated to explain the error and so the error may appear large relative to the readings but since the offset is very small in reality the error due to it is also small.
The purple circle is the circumference of the circular disc of which the vernier scales are a part.
Measured and Expected/True value
The value that is measured by us is the main scale marking which coincides with the zero of the vernier scales whereas the true/expected value is the marking that coincides with the zero of the vernier scale if the axis of the main scale and vernier scale were the same.(that is no offset is the ideal case)
The line along which the vernier scales' axis offsets is named the line of offset.(l)
The error in the measured value and the true value is due to the offset and the position of the vernier scales with respect to the line of offset.
Talk about the error due to offset
From the figure given bellow :

*

*The outer markings are of the main scale.

*The purple lines represent extended the markings on the vernier scales.Here only 0 and 10 of both scales are shown.

*The measured value at 300
is different from the true value since the vernier's zero marking
points to marking 301.

*similarly the vernier 2 measures at 142 instead of 120.

*This error in vernier 1 and vernier 2 is unchanged if the main scale is rotated, since only the markings change, the line of offset is at the same angle with vernier scales.  When
finding any angle, we take the difference between two measured
values at different positions of the telescope. This should not
affect the vernier scale and therefore the error in both measured
values will be equal resulting in no error due to offset in the measured angle which obtained by taking the difference of the readings.

*In the case where the vernier table is rotated to measure the angle, the
mean of the angles measured by vernier 1 and vernier 2 will minimise
the systematic error in measurement but it will not eradicate it completely.

Conclusion
The systematic error in measurements from V1 and V2 will be completely erased if difference of any two measurements is taken which we always do while measuring angles.
In the case where we rotate the vernier table to measure the angle (which we don't need to do), the systematic error can be reduced but not removed by taking the mean of the two measurements of V1 and V2.
